The invention relates to control of thyristor-controlled AC loads in general, and more particularly to control where series-connected thyristors are used.
Phase angle firing of thyristor switches is used to provide an adjustable voltage to an AC load such as a motor or a high voltage controlled rectifier circuit. The variable delay in firing the SCR's with respect to a reference voltage is used to provide variable voltage to the load. In high voltage applications, series-connected thyristors are used to obtain the necessary voltage support capability for the switch. Where two or more thyristors are connected in series, special management of the firing angle control for the thyristor switch is required to assure proper operation.
Because of the particular point at which conduction has started with respect to the phase voltage, the current will stop prior to simultaneous firing of the next pair of thyristors. This causes a current discontinuity. This current discontinuity is a function of the load and when the overlap situation is initiated. Current discontinuity is more apt to occur where the electrical load has a unity power factor. When the load current becomes discontinuous, the thyristors which were ON at this moment tend to commutate off or to recover while supporting reverse voltage. With series-connected thyristors forming a single overall switch combination, it is expected that the thyristors in the string will equally share the voltage developed across the switch element. It has been observed that under gate firing signals applied to such a string of thyristors the recovery process has a pronounced adverse effect upon the distribution of voltage among the series-connected thyristors. The resulting imbalance can result in excessive overvoltage on any one individual thyristor of the string, thereby causing failure.